This was a case control study of physician-diagnosed asthma, occupation, industry, and workplace environmental exposures designed to evaluate the hypothesis that a substantial component of the asthma burden in a low income, urban, largely minority population was due to occupational factors. The study design addressed a variety of methodologic challenges including healthy worker effects, difficulty contacting and recruiting this potentially high risk population, large numbers of potential etiologic agents, mixed exposures, small workplaces, and low absolute incidence of occupational asthma.